Thread: converting a 9.9 hp a 15 hp Yamaha

I did all the searchs, found out a lot. but it seemed nobody had the rest of the story!

To make a Yamaha 9.9hp 2 stroke a 15 hp.
Install a 15 hp reed pack, it has 6 reed not 4. This is simple, it is right behind the carb.
That helps a lot but does not get you all the way there. the next thing you need to do is put a new 15 hp carb on it. the 15 hp carb has a 19mm air venturi and the 9.9 only has a 15mm, 20% more air to a 15. All the jets in the carb are the same, however the brass nut thing that holds the bowl on also has a bigger hole to allow more fuel into the bowel.
This cost about $260. and it will get you to 21mph vs 16.5mph in a 14 ft aluminum jon boat with 2 big men and gear. (ps you will also need to go to a 11 pitch prop to get the speed)

I have also heard that pre 82 mercs and evinrudes could be converted, but I never found out details.

I never fish 9.9 limit lakes and I am not trying to get one over on the laws. I just happen to have a 9.9 and would rather hop it up that get a new motor.

I have to admit a 15 hp out of 79lb machine is sure sweet.

With most 4 strokes you must change the cam so it can get much more complex and costly.

Did you also compare the exhaust systems? Usually they have to be changed too because if you put more in you have to be able to get more out. Some years ago I looked into upgrading a 9.9 Johnson. It would have cost about $800. It wasn't worth it.

Might want to look at a Nissan and try this, a lot cheaper.

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There is always the rest of the story! by that I am saying there are lots of things that can be done to make a motor faster, but I actually own a 15hp and 9.9 and I took each one apart and compared the differances. they are the exact same motor with several different parts. Changing a throttle cam to make a motor 5 more hp just does not seem reasonable, there should be an adjustment to the existing one to tweek it up.
To hop up old Hurricane 10hp we welded up the exhaust ports to a smaller hole and added headers, (maybe that is why I cannot hear today). We ran those 10-to 12 K and they would fly. I had one on a old sears gamefisher and would go across back bay in no time.

The nissans are made as 15 horses and governed down to 9.9 by a throttle cam that opens the carb about 75 percent